System interconnection protective device for non-utility generation equipment

ABSTRACT

The isolation operation of a non-utility generation system (7) interconnected to a power system enable to be reliably detected in the non-utility generation system, providing a transfer breaker. When the frequency change rate of the non-utility generation system (7) has a positive value, the advanced reactive power of the non-utility generation system (7) is increased. When the frequency change rate of the non-utility generation system (7) has a negative value, the delayed reactive power of the non-utility generation system (7) is increased. Further, the gain of the power control section (9) of the non-utility generation system (7) is adjusted in accordance with the frequency change rate. As the gain is thus adjusted, the voltage of the non-utility generation system (7) changes. This change in the voltage is detected. In accordance with the voltage change, the non-utility generation system (7) is disconnected from the system power supply.

TECHNICAL FIELD

The present invention relates to an apparatus for protecting anon-utility generation system, such as a solid-waste burning generationsystem, a cogeneration system, a fuel battery system or a solargeneration system, to be interconnected to a power system.

BACKGROUND ART

Hitherto, general users have been utilizing an interconnection system ofthe type shown in FIG. 1 to interconnect a power system and anon-utility generation system such as a cogeneration system. That is, inthe upper substation 60, the transformer 2 lowers the voltage of thepower system 1. The power with its voltage thus lowered is supplied viathe breaker 3 to the houses of general users. In the house of anygeneral user, the power is supplied through a breaker 4 to a load 5.

On the other hand, in the non-utility generation system, the breaker 6connects the output of the AC generator 7 and the power system 1. Thepower output from the AC generator 7 is controlled by regulating themagnetic-field winding 8 of the AC generator by means of an automaticvoltage regulator (AVR) 9. The output frequency of the AC generator 7 iscontrolled as a speed governor 11 adjusts the power of the engine 10,which drives the AC generator 7.

A fault-detecting means is provided. A rectifier 12 detects the outputcurrent of the generator 7. An error-detecting circuit 13 detects ananomalous current on the basis of the relationship between the outputcurrent and output voltage of the generator 7. The circuit 13 suppliesan error signal to a fault trip circuit 20, which opens the breaker 6.

A protective means is provided. A rectifier 14 is connected to theoutput of the breaker 6 (namely, at the side of the substation 60), andan over-current relay (OC) 19 drives the fault trip circuit 20. When thepower system 1 has a trouble, particularly when it is blocked because,for example, the breaker 3 opens, the output power of the AC generator 7is supplied to the load 5. Consequently, the frequency and the voltagecome to have an anomalous value. This event is detected by anunder-frequency (UF) relay 15, an over-frequency (OF) relay 16, anover-voltage (OV) relay 17, an under-voltage (UV) relay 18, and thelike. The fault trip circuit 20 receives the detection signals fromthese relays and generates a trip command to the breaker 6 and opens thebreaker 6. Hence, the load 5 can be protected, and the breaker 3 can beclosed again. When a trouble occurs in the power system 1, opening thebreaker 3, the output power of the AC generator 7 and the power requiredby the load 5 may have each an active component and an idle componentwhich are equal to each other. If this case, both the frequency and thevoltage change little, and none of the relays 15 to 19 operate.Therefore, the generator 7 continues to operate, causing a so-calledisolated operation (islanding). As a consequence, an accident may takeplace, possibly preventing the breaker 3 from being closed again.

For the purpose of preventing such an isolated operation, a transferbreaker 61 is connected by a leased line to the substation 60, forperforming transfer breaking on the breaker 6. The transfer breaker 61supplies a break signal to the breaker 6, thereby opening the breaker 6,when it detects a signal which indicates that breaker 3 is opened in theupper substation 60.

The transfer breaker 61 needs to be used if the upper substation 60 isvery far or if many houses receive power from the substation 60. If usedin a non-utility generation system of medium capacity, whose output isonly hundreds of kilowatts, the transfer breaker will increase the costvery much, scarcely resulting in a practical advantage in systeminterconnection.

If the upper substation 60 is very far or if many houses receive powerfrom the upper substation 60, the transfer breaker 61 must be used,because it does not much increase the cost of the entire power systemand does results in a practical advantage in system interconnection.

However, if the transfer breaker 61 is provided in a non-utilitygeneration system of medium capacity whose output is only hundreds ofkilowatts, the cost will much increase. In such a non-utility generationsystem, it scarcely achieves a practical advantage in systeminterconnection.

The object of the present invention is to provide an apparatus forprotecting a non-utility generation system to be interconnected to apower system, which can reliably detects the isolated operation of thenon-utility generation system connected to a power system, without usinga transfer breaker which is an expensive device.

DISCLOSURE OF INVENTION

The object described above can be attained by a apparatus for protectinga non-utility generation system to be interconnected a power systemwhich is connected by a breaker to a power system and which has avoltage control system, said apparatus comprising:

a frequency detector for detecting an output frequency of thenon-utility generation system;

a frequency-change rate detector for detecting a rate at which thefrequency detected by the frequency detector changes;

arithmetic means for calculating a change reference for a voltage orreactive power output by the non-utility generation system, from thefrequency change rate detected by the frequency-change rate detector;

first control means for increasing an advanced reactive power of thenon-utility generation system or decreasing the output voltage of thenon-utility generation system, when it is determined from the changereference that the frequency change rate has a positive value;

second control means for increasing a delayed reactive power of thenon-utility generation system or increasing the output voltage of thenon-utility generation system, when the frequency change rate has anegative value;

gain adjusting means for adjusting a gain of a power control section ofthe non-utility generation system, in accordance with the frequencychange rate; and

protecting means for detecting a frequency change, which increases asthe voltage of the non-utility generation system changes, and fordisconnecting the non-utility generation system from the system bus linein accordance with the frequency change detected. The above-mentionedobject can achieved also by an apparatus for protecting aninterconnection system of a non-utility generation system which isconnected by a breaker to a power system, said apparatus comprising:

a frequency change detector for detecting changes in an output frequencyof the non-utility generation system;

control means for outputting a control signal to the non-utilitygeneration system, thereby to control a reactive power, a preset outputvoltage, an output voltage, an output voltage phase or an output currentphase of the non-utility generation system;

reactive power-change rate detector for detecting a rate at which areactive power of the non-utility generation system changes;

voltage-change rate detecting means for detecting a rate at which anoutput-voltage reference of the non-utility generation system changes;

frequency-change increasing means for changing an output of thenon-utility generation system, thereby to increase the frequency change,when the reactive power-change rate detecting means and thevoltage-change rate detecting means detect a change in the frequency ofthe non-utility generation system; and

operating mode setting means for setting an operating mode of thenon-utility generation system when the reactive power-change ratedecreases as the frequency-change increasing means increases thefrequency change.

Further, the object described above can be attained by an apparatus forprotecting a non-utility generation system which is connected by abreaker to a power system and which has a voltage control system, saidapparatus comprising:

a frequency detector for detecting an output frequency of thenon-utility generation system;

a frequency-change rate detector for detecting a rate at which thefrequency detected by the frequency detector changes;

low-speed response reactive power control means for detecting a reactivepower of the non-utility generation system, said low-speed responsereactive power control means being controlled by a first voltagereference to change the reactive power to a desired value;

high-speed response reactive power control means controlled by areactive-power change reference and a second voltage change, saidreactive-power change reference advancing the reactive power when saidfrequency-change rate detector detects that the frequency change ratehas a positive value, and delaying the reactive power when saidfrequency-change rate detector detects that the frequency change ratehas a negative value, and said second voltage change having beenobtained by comparing the reactive power and the reactive power change;

voltage control means for controlling an output voltage of thenon-utility generation system in accordance with a third voltagereference obtained from the first voltage reference for controlling thelow-speed response reactive power control means and the second voltagereference for controlling the high-speed response reactive power controlmeans; and

protective means for disconnecting the non-utility generation systemfrom a bus line in accordance with the reactive power change and alsowith the second voltage reference for controlling the high-speedresponse reactive power control means.

Still further, the object described above can be achieved by an aapparatus for protecting an interconnection system of a non-utilitygeneration system which is connected by a breaker to a power system,said apparatus comprising:

a frequency detector for detecting an output frequency of thenon-utility generation system;

a frequency-change rate detector for detecting a rate at which thefrequency detected by the frequency detector changes;

a function circuit for calculating a voltage-change reference from thefrequency change rate detected by the frequency-change rate detector andfor controlling the non-utility generation system to increase anadvanced reactive power of the non-utility generation system or decreasethe output voltage of the non-utility generation system when it isdetermined from the voltage change reference that the frequency changerate has a positive value, and to increase a delayed reactive power ofthe non-utility generation system or increase the output voltage of thenon-utility generation system when it is determined from the voltagechange reference that the frequency change rate has a negative value;

an active power detector for detecting an active power of thenon-utility generation system;

voltage-change reference correcting means for increasing thevoltage-change reference output from the function circuit, thereby tosufficiently increase the output frequency of the non-utility generationsystem when the active power detected by the active power detector issmall; and

a protective device for detecting a frequency change increasing as thevoltage of the non-utility generation system changes and fordisconnecting the non-utility generation system from a bus line.

Moreover, the above-mentioned object can be achieved by an apparatus forprotecting an interconnection system of a non-utility generation systemwhich is connected by a breaker to a power system, said apparatuscomprising:

a frequency detector for detecting an output frequency of thenon-utility generation system;

a frequency-change rate detector for detecting a rate at which thefrequency detected by the frequency detector changes;

a group of capacitors which are power capacitors;

input breaker means for disconnecting the group of capacitors from thenon-utility generation system when the frequency change rate detected bythe frequency-change rate detector has a positive value and connectingthe group of capacitors to the non-utility generation system when thefrequency change rate detected by the frequency-change rate detector hasa negative value; and

a protective device for detecting a frequency change increased as thereactive power changes when the group of capacitors is connected ordisconnected, and for disconnecting the non-utility generation systemfrom a base line in accordance with the frequency change detected.

Furthermore, the above-mentioned object can be attained by an apparatusfor protecting a non-utility generation system which is connected by abreaker to a power system, said apparatus comprising:

a frequency detector for detecting an output frequency of thenon-utility generation system;

a frequency-change rate detector for detecting a rate at which thefrequency detected by the frequency detector changes;

a function circuit for calculating a voltage-change reference from thefrequency change rate detected by the frequency-change rate detector andfor controlling the non-utility generation system to increase anadvanced reactive power of the non-utility generation system or decreasethe output voltage of the non-utility generation system when it isdetermined from the voltage change reference that the frequency changerate has a positive value, and to increase a delayed reactive power ofthe non-utility generation system or increase the output voltage of thenonutility generation system when it is determined from the voltagechange reference that the frequency change rate has a negative value;

a group of capacitors which are power capacitors,

input breaker means for disconnecting the group of capacitors from thenon-utility generation system when the frequency change rate detected bythe frequency-change rate detector has a positive value and connectingthe group of capacitors to the non-utility generation system when thefrequency change rate detected by the frequency-change rate detector hasa negative value; and

a protective device for detecting a frequency change increased as thereactive power changes when the group of capacitors is connected ordisconnected, and for disconnecting the non-utility generation systemfrom a base line in accordance with the frequency change detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a conventional apparatus forprotecting a non-utility generation system;

FIG. 2 is a block diagram showing an apparatus for protecting anon-utility of generation system, which is a first embodiment of thepresent invention;

FIG. 3 is a diagram to the block diagram of the embodiment shown in FIG.2;

FIG. 4 is a graph explaining the operation of the embodiment shown inFIG. 2;

FIG. 5 is a diagram explaining the operation of the embodiment shown inFIG. 2;

FIG. 6 is a diagram explaining the operation of the embodiment shown inFIG. 2;

FIG. 7 is a characteristic diagram illustrating the operation of thefunction circuit shown in FIG. 2;

FIG. 8 is a block diagram showing an apparatus for protecting anon-utility generation system, which is a seventh or eighth embodimentof the present invention;

FIG. 9 is a diagram showing the phase circuit shown in FIG. 8;

FIG. 10 is a block diagram showing an apparatus for protecting anon-utility generation system, which is a ninth or tenth embodiment ofthe present invention;

FIG. 11 is a block diagram depicting only one part of an apparatus forprotecting a non-utility generation system, which is an eleventhembodiment of the present invention;

FIG. 12 is a block diagram illustrating an apparatus for protecting anon-utility generation system, which is a twelfth embodiment of theinvention;

FIG. 13 is a diagram supplemental to the block diagram of the embodimentshown in FIG. 12;

FIG. 14 is a diagram explaining the operation of the embodiment shown inFIG. 12;

FIG. 15 is a graph explaining the operation of the embodiment shown inFIG. 12;

FIG. 16 is a graph explaining the operation of the embodiment shown inFIG. 12;

FIG. 17 is a graph explaining the operation of the embodiment shown inFIG. 12;

FIG. 18 is a block diagram showing an apparatus for protecting anon-utility generation system, which is a thirteenth embodiment of theinvention;

FIG. 19 is a diagram supplemental to the block diagram of the secondembodiment of the invention;

FIG. 20 is a block diagram showing an apparatus for protecting anon-utility generation system, which is a fourteenth embodiment of theinvention;

FIG. 21 is a block diagram showing an apparatus for protecting anon-utility generation system, which is a fifteenth embodiment of theinvention;

FIG. 22 is a diagram supplemental to the block diagram of a sixteenthembodiment of the invention;

FIG. 23 is a diagram supplemental to the block diagram of a seventeenthembodiment of the invention;

FIG. 24 is a diagram supplemental to the block diagram of an eighteenthembodiment of the invention;

FIG. 25 is a block diagram showing an apparatus for protecting anon-utility generation system, which is a nineteenth embodiment of theinvention;

FIG. 26 is a diagram explaining the operation of the embodiment shown inFIG. 25;

FIG. 27 is a graph explaining the operation of the embodiment shown inFIG. 25;

FIG. 28 is a graph explaining the operation of the embodiment shown inFIG. 25;

FIG. 29 is a graph explaining the operation of the embodiment shown inFIG. 25;

FIG. 30 is a block diagram showing an apparatus for protecting anon-utility generation system, which is a twentieth embodiment of theinvention;

FIG. 31 is a block diagram showing an apparatus for protecting anon-utility generation system, which is a twenty-first embodiment of theinvention;

FIG. 32 is a diagram supplemental to the block diagram of atwenty-second embodiment of the invention;

FIG. 33 is a block diagram showing an apparatus for protecting anon-utility generation system, which is a twenty-third embodiment of theinvention;

FIG. 34 is a diagram supplemental to the block diagram of atwenty-fourth embodiment of the invention;

FIG. 35 is a block diagram depicting an apparatus for protecting anon-utility generation system, which is a twenty-sixth embodiment of theinvention;

FIG. 36 is a block diagram showing an apparatus for protecting anon-utility generation system, which is a twenty-seventh embodiment ofthe invention;

FIG. 37 is a block diagram illustrating an apparatus for protecting anon-utility generation system, which is a twenty-eighth embodiment ofthe invention; and

FIG. 38 is a diagram supplemental to the block diagram of thetwenty-fourth embodiment of the invention.

BEST MODE OF CARRYING OUT THE INVENTION

(First Embodiment)

(Structure)

FIG. 2 is a block diagram showing the first embodiment of the presentinvention. It differs from the conventional system (FIG. 1) forprotecting a non-utility generation system, in that it is constructed aswill be described below, not having a transfer breaker 38 which is anexpensive device.

As shown in FIG. 2, a frequency detector 21 detects a frequency (f) fromthe output voltage of an AC generator 7 that is a non-utility generationsystem of revolving-armature type. From the frequency thus detected, afrequency-change rate detector 30 detects a rate (df/dt) of change inthe frequency. An excessive change-rate detector 31 detects whether therate V₃₀ of change in the frequency has exceeded a preset value. Upondetecting that the rate V₃₀ has exceeded the preset value, the detector31 outputs an error signal V₃₁. The error signal V₃₁ is supplied to afault trip circuit 20. The fault trip circuit 20 gives a trip to abreaker 6, opening the electric circuit.

A reactive power detector 23 receives the output current of thegenerator 7 which a current transformer 12 has detected, and alsoreceives the output voltage of the generator 7. The detector 23 detectsa reactive power. Meanwhile, an active power detector 26 receives theoutput current of the generator 7 which the transformer 12 has detected,and also receives the output voltage of the generator 7, and detects anactive power.

An active power regulator (APR) 27 compares the active power referenceP* set by an active power reference (P*) setting device 25 with theactive power P detected by the active power detector 26. The differencebetween the active power reference and the active power is supplied to aspeed governor 11, which controls the speed of an engine 10.

A function circuit 32 is designed to receive a rate V₃₀ of change in thefrequency from the frequency-change rate detector 30 and to output areactive-power change reference ΔQ₁ *. More specifically, as shown inFIG. 5, the reactive-power change reference ΔQ₁ * causes the advancedreactive power to increase while the frequency change rate df/dt isincreasing, thereby to promote an increase in the frequency. Further thereference ΔQ₁ * causes the delayed reactive power to increase while thechange rate df/dt is decreasing, thereby to promote a decrease in thefrequency.

A reactive power regulator (AQR) 24 outputs a voltage reference V* forequalizing the reactive power Q detected by the reactive power detector23 and a new reactive power reference that is the sum of the reactivepower Q* supplied from a reactive power reference (Q*) setting device 28and the reactive-power change reference ΔQ₁ * supplied from the functioncircuit 32.

An automatic voltage regulator (AVR) 9 receives the voltage reference V*from the reactive power regulator (AQR) 24 and gives a field magnetcommand to a field magnet winding 8, thereby to control the outputvoltage of the generator 7.

The active power reference setting device 25, active power regulator(APR) 27, speed governor 11 and engine 10 constitute a speed controlloop. The reactive power reference (Q*) setting device 28, reactivepower detector 23 and reactive power regulator (AQR) 24 constitute areactive power control loop. The voltage reference V*, which is theoutput of the reactive power regulator (AQR) 24, and the automaticvoltage regulator (AVR) 9 constitute a voltage control loop.

A level detector 34 detects a plurality of levels from the frequencychange rate V₃₀. A circuit 36 changes over the function in the functioncircuit 32 after a timer 35 has detected the level, only for a time.

A level detector 37 detects whether or not the frequency detected by thefrequency detector 21 has changed from the rated value. If the detector37 detects this, a changeover circuit 38 operates to decrease the gainof the function circuit 32.

(Operation)

The basic operation of the first embodiment described above will beexplained, with reference to FIGS. 3 to 6. The active power SP andreactive power SQ, both supplied to a power system 1 are given asfollows:

    SP=P-P.sub.L

    SQ=Q-Q.sub.L

where P is the active power output by the generator 7, Q is the reactivepower, P_(L) is the active power a load 5 needs, Q_(L) is the reactivepower, as is shown in FIG. 3.

Here, I is the inductance I between the generator 7 and the system, V isthe voltage applied on the load 5, and f is the frequency.

In most cases, the voltage V on the load 5 and the frequency f scarcelychange even if a breaker 3 is opened while SP and SQ are almost zero(0). Thus, relays 15 to 19 are not detected, an isolated operationcontinues.

However, the phase of the power system 1 and the phase of the load 5gradually deviate from each other. If the breaker 3 is closed again, agreat accident may arise. Hence, the breaker 3 cannot be closed again.This will impair the safety operation of the power distribution system.

The voltage during the isolated operation is determined as P=V² /R. Onthe other hand, the frequency f during the isolated operation isdetermined by Q=(V² ωC)-(V² /ωL). The frequency f, in particular,increases when the reactive power the generator 7 supplies advances withrespect to the reactive power Q_(L) the load 5 needs. In this case, thecurrent i_(C) in a capacitor C increases and the inductance currenti_(L) decreases, whereby the reactive power changes to become balanced.

When the reactive power the generator 7 supplies delays with respect tothe reactive power Q_(L) the load 5 needs, the frequency f decreases andthe inductance current i_(C) increases. The capacitor current i_(C)therefore decreases, and the reactive power becomes balanced.

When the isolated operation starts while SP=0 and SQ≠0, the frequency fapproaches either f₁ or f₂ while changing, as shown in FIG. 4, after thesystem has been disconnected (t₀), as has been actually observed andconfirmed by simulation.

In FIG. 4, f₁ is the value the frequency f has when SQ is advanced alittle, and f₂ is the value the frequency f has when SQ is delayed alittle. +Δf and -Δf, both shown in FIG. 4, are levels at which therelays 15 to 19 can detect isolated operation.

FIG. 5 is a diagram explaining the operational advantage of theembodiment shown in FIG. 2. In FIG. 5, f is the frequency the frequencydetector 21 has detected, df/dt is the frequency change rate thefrequency-change rate detector 30 has detected, and ΔQ₁ * is the outputof the function circuit 32.

If f changes as shown in FIG. 5, df/dt will have a waveform, whichadvances in phase by 90°.

When df/dt>0, the frequency is increasing. In this case, the functioncircuit 32 outputs ΔQ₁ * which is advanced, and the frequency f furtherincreases. When df/dt<0, the frequency f is decreasing. In this case,the function circuit 32 outputs ΔQ₁ * which is delayed, and thefrequency f further decreases. This positive feedback operationincreases the rate of frequency change as is illustrated in FIG. 6, andthe excessive change-rate detector 31 detects an anomalous frequency oran excessive rate of frequency change. This makes it possible to detectthe isolated operation and protect the system, without using a transferbreaker (e.g. breaker 61 shown in FIG. 1) which is incorporated in theconventional system and which is an expensive device.

The basic operation has been explained. Once the isolated operation hasstarted while SP=0 and SQ=0, it takes some time until the change in thefrequency is enhanced through the positive feedback. In some cases, itmay difficult to detect the isolated operation within three seconds orless after the circuit is closed again.

In order to shorten this time, the gain of the function circuit 32 isincreased in the present invention, thereby increasing the positivefeedback value. Despite of the scheme, it remains possible that thechange in a change in df/dt which occurs while the generation systemremains connected with the system power supply in normal state maytrigger an excessive change in the reactive power.

To solve this problem, the function circuit 32 shown in FIG. 2 has, forexample, a special function A shown in FIG. 7.

Namely, when the isolated operation starts while SQ=0, df/dt isextremely small in the initial phase, falling within a range fromdf/dt(1) to df/dt(3). A voltage-change reference V₁ * that has increasedthe gain within this range is output, thereby increasing the positivefeedback value. The voltage-change reference ΔV₁ * works as a kind of atrigger signal.

The frequency change rate df/dt(2) is at a value close to the maximumvalue which df/dt may have while the generation system remains connectedto the system power supply. The gain is decreased for the range fromdf/dt(1) to df/dt(2) (or ΔV₁ * is limited, setting a function B),thereby preventing an excessive change in the reactive power (anexcessive voltage change). The gain increases again if df/dt fallsoutside the range from df/dt(2) to df/dt(4). The change in the frequencyis thereby increased during the isolated operation. Minorcharacteristics such as A and B may be varied in accordance with thecondition of the system.

(Advantages)

As described above, the first embodiment of the invention can provide anapparatus for protecting a non-utility generation system, which performsa stable operation at high speed and in which a trigger signal having asubstantially constant value ΔV₁ * is used, shortening the time requiredto detect isolated operation and suppressing the change in the voltageof the interconnection system.

(Second Embodiment)

A possible isolated operation is detected if the change rate df/dt thelevel detector 34 (FIG. 2) has detected is small. When the rate df/dtreaches the first level, the changeover circuit 36 increases the gain ofthe function circuit 32 and the gain is monitored, only for the time bythe timer 35. For example, the gain is increased from characteristic G₁to characteristic G₂, both shown in FIG. 7, and is monitored. When thelevel detector 34 detects that the gain reaches the second level, thegain is further increased from G₂ to G₃, both shown in FIG. 7, for thetime preset by the timer 35. Thus, the isolated operation can be fastdetected and, in addition, the gain can be prevented from becomeexcessive even if the rage in change of the system frequency increasesto excess due to an accident. The system can therefore remain stable.The gain recovers to G₁, by virtue of the timer 35, when the systemfrequency ceases to change.

This control of the gain can be accomplished, achieving the same effect,also by varying the voltage-change reference ΔV₁ *.

(Third Embodiment)

In the generation system shown in FIG. 2, when the change in thefrequency becomes excessively large during the isolated operation, theelectric devices, such as the electric motor, connected to the load mayhave a trouble. To prevent such a trouble, the level detector 37 detectswhether the frequency f is excessively large or excessively small, andthe changeover circuit 38 decreases the gain of the function circuit 32and the output limit is reduced. The positive feedback value is therebydecreased, suppressing the change in the frequency. This enhances thereliability and safety of the apparatus.

(Fourth Embodiment)

When the load 5 shown in FIG. 2 undergoes isolated operation, in whichthe ratio of the motor to the load is high, the change P in active powerand the change Q are small in comparison with the change V in the outputvoltage of the generator. Hence, there is the trend that the rate df/dtof frequency change would not increase.

If this is the case, the apparatus may be modified to one shown in FIG.8. The apparatus shown in FIG. 4 differs from the apparatus of FIG. 2 inthat a voltage-change reference (ΔV₁ *) detecting circuit 42, a reactivepower change (ΔQ) detecting circuit 41, a ΔQ/AV₁ * detecting circuit 42,a level detector 43 and an OR circuit (logic sum circuit) 44 areadditionally provided.

No components equivalent to the level detectors 34 and 37 and timer 35,level detectors 36 and 38 are not shown in FIG. 8. In practice, however,the level detectors 34 and 37 and timer 35, level detectors 36 and 38may be required in many cases.

The voltage-change reference detecting circuit 40 detects ΔV₁ *, and thereactive power change detecting circuit 41 detects ΔQ. These valuesΔV₁ * and ΔQ are input to the ΔQ/ΔV₁ * detecting circuit 42. The circuit42 detects the ratio of ΔQ to ΔV₁ *. The ratio ΔQ/ΔV₁ * detected by theΔQ/ΔV₁ * detecting circuit 42 is input to the level detector 43. Thelevel detector 43 detects that the ratio ΔQ/ΔV₁ * has decreased. Thatis, the detector 43 detects the isolated operation. The output of thedetector 43 is supplied via the OR circuit 44 to the fault trip circuit20, thereby disconnecting the generator 7 from the system.Alternatively, the output of the excessive change-rate detector 31 issupplied via the OR circuit 44 to the fault trip circuit 20, therebydisconnecting the generator 7 from the system. As long as the apparatusis connected to the generation system, the power supply impedance islow. The ratio ΔQ/ΔV₁ * is therefore large, but becomes smallparticularly when the load on the electric motor is great during theisolated operation. This event is utilized in the apparatus.

Needless to say, the change ΔV in the output voltage of the generatormay be used in place of the voltage-change reference ΔV₁ * shown in FIG.8. In this case, the apparatus operates in exactly the same way.

(Fifth Embodiment)

The fifth embodiment differs from the fourth embodiment in that anexcessive change detecting circuit 45 and a gain control (limit) circuit46 are additionally provided as shown in FIG. 8. When the excessivechange detecting circuit 45 detects that the output the reactive powerchange detecting circuit 41 detects ΔQ of the reactive power changedetecting circuit 41 has exceeded the preset value, the gain control(limit) circuit 46 decreases the gain of the function circuit 32 orlimits the output thereof. The change in the voltage-change referenceΔV₁ * is thereby suppressed. As a result, the change in the reactivepower is limited.

The apparatus operates in almost the same way if ΔQ/Δt is detected,instead of using the reactive power change detecting circuit 41.

(Sixth Embodiment)

The sixth embodiment differs from the fourth and fifth embodiments inthat an off circuit 47, a switch circuit 48 and an auxiliary contact 3aare additionally provided as shown in FIG. 8. The isolated operation isdetected at the auxiliary contact 3a , thus determining that the breaker3 has been opened. The switch circuit 48 connected in series between theoutput of the voltage-change reference setting device 33 and the inputof an automatic voltage regulator 9 is thereby opened, opening thepositive feedback circuit. The breaker 3 is opened and disconnected fromthe system power supply from the system. At the same time, the positivefeedback loop is opened, constituting a stable voltage control loop. Thegenerator 7 can be used as an independent power supply.

(Seventh Embodiment)

The seventh embodiment differs from the fourth to sixth embodiments inthat a phase-characterizing circuit 39 is additionally provided as shownin FIG. 8. More specifically, the circuit 39 is connected between theinput of the frequency detector 21 and one end of the breaker 6. Thephase-characterizing circuit 39 is designed to advance the phase as thefrequency increases, as long as the frequency remains in the vicinity ofits rated value. Its characteristic is represented by plotting thefrequency and the phase on the abscissa and the ordinate, respectively.

Hence, the circuit 39 can detect frequency changes in a magnified form,thus promoting the positive feedback in the vicinity of the ratedfrequency and shortening the time required to detect the isolatedoperation. When the frequency changes more greatly, the phasecharacteristic is inverted, automatically suppressing the positivefeedback. An excessive change in the frequency can thereby prevented.Needless to say, the phase-characterizing circuit 39 thus designed canperform software process after it has detected the frequency f.

(Eighth Embodiment)

The phase characterizing circuit 39 shown in FIG. 8 comprises band-passfilters 391 and 392 and an adder circuit 393 as illustrated in FIG. 9.The band-pass filter 391 has a resonance point at a frequency slightlyhigher than the rated frequency. The band-pass filter 392 has aresonance point at a frequency slightly lower than the rated frequency.The adder circuit 393 adds the outputs of the band-pass filters 391 and392. Thus constructed, the circuit 39 has the gain characteristic andthe phase characteristic, which are shown in the upper and lower partsof FIG. 9.

The phase characterizing circuit 39 can be provided at the output of thefrequency detector 21 by means of software.

(Ninth Embodiment)

The ninth embodiment differs from the embodiment of FIG. 2 in that thereactive power regulator 24 is replaced by a low-speed response reactivepower control circuit 50, a voltage reference (V_(A) *) setting device51, a ΔQ detecting circuit 52, a high-response reactive power controlcircuit 53, a voltage-change reference (ΔV_(B) *) setting device 54, anadder circuit 55 and a ΔQ/ΔV_(B) * detector 56, as is illustrated inFIG. 10. Although the level shifters 34 and 37, timer 35 and change-overcircuits 36 and 38 are not shown in FIG. 10, these components may berequired in many cases.

In this embodiment, the outputs of the reactive power reference settingdevice 28 and reactive power detector 23 are compared, and thedifference between the outputs is input to the low-speed responsereactive power control circuit 50. The low-speed response reactive powercontrol circuit 50 generates an output only when the frequency is in alow range. This is because its gain is large when the frequency is inthe low range. As a result, the voltage reference setting device 51outputs a voltage reference (V_(A) *) which will control the reactivepower for about tens of seconds. The voltage reference is input to oneinput terminal of the adder circuit 55.

Meanwhile, the output of the reactive power detector 23 is input to thereactive-power change detecting circuit 52. The circuit 52 detects achange in the reactive power. The change in the reactive power iscompared with the output of the function circuit 32. The differencebetween the values compared is input to the high-response reactive powercontrol circuit 53.

The high-response reactive power control circuit 53 has a large gainonly when the frequency is in a high range. Therefore, it generates anoutput only when the frequency is in the high range. The voltage-changereference (ΔV_(B) *) setting device 54 generates an output ΔV_(B) *. Theoutput ΔV_(B) * is input to the other input terminal of the addercircuit 55 to control the reactive power for about 0.5 seconds.

The adder circuit 55 adds the voltage-change reference ΔV_(B) * and thevoltage reference V_(A) *, thus generating a voltage chance referenceΔV₁ *. The voltage chance reference ΔV₁ * is compared with the change Vin the output voltage of the generator. The difference between thevalues compared is input to the automatic voltage regulator 9.

The reactive power change output from the reactive-power changedetecting circuit 52 and the voltage-change reference ΔV_(B) * outputfrom the voltage-change reference setting device 54 are input to theΔQ/ΔV_(B) * detector 56. The ratio of ΔQ to ΔV_(B) * (ΔQ/ΔV_(B) *) isinput to the level detector 43. When the level detector 43 detects thatthe ratio ΔQ/ΔV_(B) * has decreased, or detects the isolated operation,it generates an output V₃₁. This output, or a trip command, is suppliedto the fault trip circuit 20. Hence, while the generation system remainsinterconnected to the system power supply, ΔQ can be controlled at highspeed in accordance with ΔQ₁ *. ΔQ changes very little during theisolated operation particularly when the load on the electric motor issmall. Therefore, ΔQ scarcely changes even if ΔV_(B) * changes greatly.This makes it possible to detect the isolated operation by detecting adecrease in the ratio ΔQ/ΔV_(B) *.

(Tenth Embodiment)

In the ninth embodiment described above, the voltage-change reference(ΔV₁ *) setting device 33 is not provided at the output of the functioncircuit 32. In the tenth embodiment, the voltage-change referencesetting device 33 is provided and the voltage-change reference ΔV₁ * issupplied to an input terminal of the adder circuit 55.

While the generation system remains interconnected to the system powersupply, ΔV₁ * is fed forward and ΔV_(B) * is applied in a feedback loopto control the change in reactive power. When ΔQ becomes zero (0) duringthe isolated operation in which mainly the load on the motor is driven,ΔV_(B) * and ΔV₁ * are added, increasing ΔQ. In this case, ΔQ/ΔV_(B) *decreases.

(Eleventh Embodiment)

The generation system incorporated in the embodiments described abovemay be either a rotary machine such as an AC generator or a staticsystem such as an inverter or an SVC (Static Var Compensator). Inpractice, both types of generation systems are used together. Thecontrol of a rotary machine is delayed by about 0.5 seconds due to theresponse time of the field magnet. A static system responds very fast;its response time is negligibly short. To use both types of generationsystems, it is therefore necessary to delay the control of the staticsystem, thereby to operate the static system in synchronism with therotary machine. Thus, as shown in FIG. 11, a delay circuit 57 delays ΔV₁*, thereby providing a voltage-change reference, and a delay circuit 58delays ΔQ₁ *, thereby providing a reactive-power change reference, whichis supplied through the reactive power regulator 24. This makes itpossible to use a static converter.

(Twelfth Embodiment)

(Structure)

FIG. 12 is a diagram illustrating the twelfth embodiment of the presentinvention. This embodiment differs from the conventional (FIG. 1)apparatus for protecting a non-utility generation system in that it hasno transfer breaker 38 which is an expensive component and that it isconstructed as will be described below.

As shown in FIG. 12, a frequency (f) detector 121 detects the frequencyfrom the output of an AC generator 107. A frequency-change rate (df/dt)detector 130 detects a frequency change rate V₃₀.

An excessive frequency-change rate (df/dt) detector 131 detects whetheror not the frequency-change rate V₃₀ has reached or exceeded a presetvalue. Upon detecting that the rate V₃₀ has reached or exceeded a presetvalue, the detector 131 outputs an error signal V₃₁, which is suppliedto a fault trip circuit 120. The fault trip circuit 120 supplies a tripsignal to a breaker 106, opening the electric circuit.

A reactive power detector 123 receives the output current of thegenerator 107, which has been detected by a current transformer 112, andthe output voltage of the generator 107, and detects an reactive powertherefrom. An active power detector 126 receives the output current ofthe generator 107, which has been detected by a current transformer 112,and the output voltage of the generator 107, and detects an active powertherefrom.

Meanwhile, an active power regulator (APR) 127 compares the active powerreference P* set by an active power reference (P*) setting device 125with the active power P detected by an active power detector 126. Thedifference between the active power reference and the active power issupplied to a speed governor 111, which controls the of an engine 110.

A function circuit 132 is designed to receive a rate V₃₀ of change inthe frequency from the frequency-change rate detector 30 and to output avoltage change rate ΔV*. The voltage change rate ΔV* serves to decreasethe output voltage of the generator 107, promoting an increase of thefrequency, while the frequency change rate remains positive (that is,while the frequency is increasing). The rate ΔV* also serves to increasethe output voltage of the generator 107, promoting a decrease of thefrequency, while the frequency change rate remains negative (that is,while the frequency is decreasing). The voltage change rate ΔV* iscorrected by a voltage-change reference correcting means 133 on thebasis of the active power P detected by the active power detector 126.The means 133 outputs a corrected voltage-change reference ΔV*'.

The voltage-change reference correcting means 133 may be designed asshown in FIG. 13. Namely, it may have a gain K which is a function ofthe active power P detected and may multiply the gain K by the voltagechange rate ΔV*, thereby to obtain a corrected voltage-change referenceΔV*'. The means 133 may of course be of any other structure, so long asit can correct the voltage change rate ΔV*. For example, it may bedesigned to act on the function of the function circuit 132, eitheraltering or correcting the function.

A reactive power regulator (AQR) 124 outputs a voltage reference ΔVQ*for equalizing the reactive power reference Q* supplied from a reactivepower reference setting device 128 and the reactive power detected bythe reactive power detector 123.

An automatic voltage regulator (AVR) 109 receives the voltage referenceV* from a voltage-reference setting device (90R) 129, the voltagereference ΔVQ* from the reactive power regulator 124, and the correctedvoltage-change reference ΔV*' from the voltage-change referencecorrecting means 133. Using these inputs, the regulator 109 regulatesthe field magnet of a filed magnet winding 108 in order to control theoutput voltage of the generator 107.

The active power reference setting device 125, active power regulator127, speed governor 111 and engine 110 constitute a speed control loop.The reactive power reference setting device 128, reactive power detector123 and reactive power regulator 124 constitute a reactive power controlloop.

The voltage-reference setting device (90R) 129, the voltage referenceΔVQ* output from the reactive power regulator 124, the correctedvoltage-change reference ΔV*' supplied from the voltage-change referencecorrecting means 133, and the automatic voltage regulator (AVR) 9constitute a voltage control loop.

(Operation)

The operation of the twelfth embodiment described above will beexplained, with reference to FIGS. 14 to 17. The active power ΔP andreactive power ΔQ, both supplied to a system power supply 1 are given asfollows:

    ΔP=P-P.sub.L

    ΔQ=Q-Q.sub.L

where P is the active power output by the generator 107, Q is thereactive power output from the generator 107, P_(L) is the active powera load 105 needs, Q_(L) is the reactive power the load 105 needs, as isshown in FIG. 14.

Here, I is the inductance I between the generator 107 and the system, Vis the voltage applied on the load 105, and f is the frequency.

In most cases, the voltage V on the load 105 and the frequency fscarcely change even if a breaker 103 is opened while ΔP and ΔQ arealmost zero (0). Thus, relays 115 to 119 are not detected, an isolatedoperation continues.

However, the phase of a system power supply 101 and the phase of theload 105 gradually deviate from each other. If the breaker 103 is closedagain, a great accident may arise. Hence, the breaker 103 cannot beclosed again. This will impair the safety operation of the powerdistribution system.

The voltage during the isolated operation is determined as P=V² /R. Onthe other hand, the frequency f during the isolated operation isdetermined by Q=(V² ωC)-(V² /ωL). The frequency f, in particular,increases when the reactive power the generator 107 supplies advanceswith respect to the reactive power QL the load 105 needs. In this case,the current i_(C) in a capacitor C increases and the inductance currenti_(L) decreases, whereby the reactive power changes to become balanced.

When the reactive power Q the generator 107 supplies delays with respectto the reactive power QL the load 105 needs, the frequency f decreasesand the inductance current iL increases. The capacitor current iCtherefore decreases, and the reactive power becomes balanced.

If ΔP=0 and ΔQ≠0, the frequency f approaches either f1 or f2 whilechanging, as shown in FIG. 15, after the system has been disconnected(t₀). In FIG. 15, f1 is the value the frequency f has when ΔQ isadvanced a little, and f2 is the value the frequency f has when ΔQ isdelayed a little. +Δf and -Δf, both shown in FIG. 15, are levels atwhich the relays 115 to 119 can detect isolated operation.

FIG. 16 is a diagram explaining the operational advantage of theembodiment shown in FIG. 12. In FIG. 16, f is the frequency thefrequency detector 121 has detected, df/dt is the frequency change ratethe frequency-change rate detector 130 has detected, and thevoltage-change reference ΔV* is the output the function circuit 32 hasgenerated.

If f changes as shown in FIG. 16, df/dt will have a waveform whichadvances in phase by 90°.

When df/dt>0, the frequency is increasing. In this case, the functioncircuit 132 outputs a voltage-decreasing command (an advanced reactivepower command), which increases the frequency f further. When df/dt<0,the frequency f is decreasing. In this case, the function circuit 132outputs a voltage-increasing command (a delayed reactive power command),which decreases the frequency f further. This positive feedbackoperation increases the rate of frequency change. The excessivefrequency-change rate detector 131 detects an anomalous frequency or anexcessive rate of frequency change. This makes it possible to detect theisolated operation and protect the system, without using a transferbreaker 138 which is incorporated in the conventional system and whichis an expensive device.

This function of increasing the frequency change results from not onlythe relation between the reactive power and the reactance load, but thechange in the active power. Further, the function takes place in thespeed control system. The active power change is determined from therelation between the voltage change and the active component of theload. Assuming that the voltage change remains unchanged, the activepower is small when the active load component is small, and is largewhen the active load component is large. When the active power changes,the load torque on the engine 110 changes, too. Hence, the speedchanges, which results in a frequency change. The frequency change issmall when the active power change is small, and is large when theactive power change is large. That is, the frequency change is smallwhen the active load component is small, and is large when the activeload component is large, provided that the voltage change remainsunchanged. Since protection is performed in accordance with the changein the frequency, it is influenced by the frequency change that is smallwhen the active load component is small. To compensate for thisinfluence, the voltage-change reference correcting means 133 isprovided. Even if the active load component is small, the correctingmeans 133 increases the voltage-change reference, increasing the voltagechange. The active power change can therefore be increase. This helps toimprove the frequency change.

In the voltage-change reference correcting means 33 having the structureshown in FIG. 13, the influence of the active load component on thefrequency change is corrected the gain K on the basis of the activepower detected. The means 33 greatly corrects the voltage-changereference ΔV*, generating a corrected voltage-change reference ΔV*'. Thevoltage-change reference ΔV*', serves to change the voltage so that asufficient frequency change may be obtained even if the active power issmall.

When the active load component is sufficiently large, the frequencychange may be excessively large. In this case, the means 133 slightlycorrects the voltage-change reference ΔV*, generating a correctedvoltage-change reference ΔV*'. This corrected voltage-change referenceΔV*' prevents the voltage change from increasing too much. Hence, thefrequency change is prevented from increasing excessively. In the meansshown in FIG. 13, the gain K and the active power P have such relationthat the gain K is large when the active power P is small, and is smallwhen the active power P is large. Thus, the correction of thevoltage-change reference achieves advantages.

Such circuits cooperate, increasing the frequency change during theisolated operation as is illustrated in FIGS. 15 to 17. This makes iteasy to detect the frequency or an anomalous rate of frequency change,thereby to detect the isolated operation.

(Advantages)

In the twelfth embodiment described above, a voltage-decreasing commandis supplied to the automatic voltage regulator 109 when df/dt>0, and avoltage-increasing command is supplied to the automatic voltageregulator 109 when df/dt<0. As a result, the output voltage of thegenerator 107 is thereby changed, and the frequency change is increased.This makes it easy to detect the isolated operation.

(Thirteenth Embodiment)

(Structure)

FIG. 18 shows the thirteenth embodiment of the invention. The thirteenthembodiment differs from the embodiment of FIG. 12 in that neither thefunction circuit 132 for outputting a voltage-change reference nor thevoltage-change reference correcting means 133 are provided and that adummy load 141 and an impedance-inputting breaker 142 are provided. Thedummy load 141 is connected to a bus line by the impedance-inputtingbreaker 142. The dummy load 141 an inductive/capacitive load. Itcomprises one inductive load and one cacitive load. Alternatively, itcomprises a plurality of inductive loads and a plurality of capacitiveloads.

The impedance-inputting breaker 142 comprises, for example, switches142a and 142b and a controller 142c, as shown in FIG. 19. The switch142a is connected to the capacitive load 141a provided in the dummy load141. The switch 142b is connected to the inductive load 141bincorporated in the dummy load 141. The controller 142c is provided toopen and close the switches 142a and 142b in accordance with afrequency-change rate (df/dt) signal.

(Operation)

As has been explained with reference to FIG. 13 in describing thetwelfth embodiment, during the isolated operation the reactive power thegenerator 107 supplies may advances with respect to the reactive powerQL the load 105 needs. In this case, the frequency f increases,increasing the current iC of the capacitor C and decreasing theinductance current iL. The reactive power changes to be balanced. Duringthe isolated operation, the reactive power the generator 107 suppliesmay delay with respect to the reactive power QL the load 105 needs. Ifso, the frequency f decreases, decreasing the current iC of thecapacitor C and increasing the inductance current iL. The reactive powerchanges to be balanced.

That is, if the reactive power the generator 107 supplies and thereactive power the load needs greatly differ from each other, thefrequency f changes greatly to balance these reactive powers. When thefrequency f changes to balance the inactive powers after the start ofthe isolated operation, it suffices to close or open the dummy load 141in order to render the reactive powers unbalanced. In the thirteenthembodiment, the dummy load 141 is usually disconnected from the busline. While df/dt>0 and, hence, the frequency is increasing, only theinductive load is connected to the bus line, thereby further increasingthe frequency f. While df/dt<0 and the frequency is decreasing, only thecapacitive load is connected to the bus line, thereby further decreasingthe frequency f. Thus, the change in the frequency is enhanced. Theexcessive frequency-change rate detector can therefore detect ananomalous frequency or an excessive rate of frequency change. This makesit possible to detect the isolated operation, without using a transferbreaker 138 which is used in the conventional system and which is anexpensive device.

Since the dummy load 141 is usually connected to the bus line, thefrequency increases when df/dt>0. When df/dt>0, only the capacitive loadis disconnected from the bus line. When df/dt<0 and the frequency isdecreasing, only the inductive load is disconnected from the bus line.However, since the dummy load 141 is always connected, it is necessaryto make the reactance of the inductive load and that of the capacitiveload substantially equal. The use of such circuits cooperate, increasingthe frequency change during the isolated operation as is illustrated inFIGS. 15 to 17. It is therefore easy to detect the frequency or ananomalous rate of frequency change, thereby to detect the isolatedoperation.

(Advantages)

In the thirteenth embodiment described above, the inductive load or thecapacitive load is connected or disconnected in accordance with thepolarity of the df/dt detected. The frequency change is therebyincreased, making it easy to detect the isolated operation.

(Fourteenth Embodiment)

(Structure)

FIG. 20 illustrates the fourteenth embodiment of the present invention.The fourteenth embodiment differs from the thirteen embodiment of FIG.18 in that a function circuit 132 is additionally provided. The circuit132 is designed to output a voltage-change reference ΔV*.

(Operation and Advantages)

As explained in describing the twelfth and thirteen embodiments, thevoltage-change reference ΔV* output from the function circuit 132 servesto enhance the change in frequency f. The connection and disconnectionof the dummy load 141, controlled by the impedance-inputting breaker142, also serves to increase the change in frequency f. Needless to say,the voltage-change reference ΔV* and the connection and disconnection ofthe dummy load 141 cooperate, easily increasing the change in frequencyf. Hence, the frequency change can be enhanced, even if thevoltage-change reference ΔV* is increased not so much. The cross currentflowing between the generator 107 and the system can be minimized whilethe generator 107 and the system remain connected.

If the output voltage of the generator 107 rises or falls due to thevoltage-change reference ΔV*, it will influence the change in frequencyf with some delay because of the delay in the voltage control loop. Onthe other hand, the connection or disconnection of the dummy load 141influences the change in frequency f without delay, because the dummyload 141 is connected or disconnected by the use of the control loop.The connection or disconnection of the dummy load 141 therefore helps tochange the output voltage of the generator 107, until the voltage-changereference ΔV* starts increasing or decreasing the output voltage of thegenerator 107.

(Fifteenth Embodiment)

(Structure)

FIG. 21 shows the fifteenth embodiment of the invention. The fifteenthembodiment differs from the twelfth embodiment of FIG. 12 in that adummy load 141 and an impedance-inputting breaker 142 are additionallyprovided. Rather, the fifteenth embodiment may be said to differ fromthe fourteenth embodiment in that a voltage-change reference correctingmeans 133 is additionally provided.

(Operation and Advantages)

The voltage-change reference ΔV* can be increased even if the activepower component is small, thereby providing a sufficiently largefrequency change, as in the twelfth embodiment. Further, thanks to thevoltage-change reference correcting means 133, the voltage-changereference ΔV* can be decreased to prevent the frequency change fromincreasing too much when the active power component is sufficientlylarge.

(Sixteenth Embodiment)

(Structure)

The load 105 may be a constant impedance load or an inductive motorload. In view of this, the sixteenth embodiment differs from theembodiments of FIGS. 12 and 21 in that a signal representing the activepower P is not input to the voltage-change reference correcting means133. Rather, as shown in FIG. 22, the signal is input to a load-ratiosetting means 143, the means 143 detects only the active power componentP' obtained by the constant impedance load, and a signal representingthe active power component P' is input to the voltage-change referencecorrecting means 133.

The load-ratio setting means 143 has, for example, a setting device143a. The value preset in the device 143a is multiplied by the signalrepresenting the active power P.

(Operation and Advantages)

In the case of an inductive motor load, the active power does not changeso much even if the output voltage of the generator 107 is changed inaccordance with the voltage-change reference ΔV*. Therefore, it cannotbe expected as in the twelfth embodiment that the active power changesas the voltage changes or the frequency changes in the speed controlsystem.

Thus, it is detected which part of the active power P detected hasresulted from the constant impedance load, and the voltage-changereference ΔV* is corrected in accordance with the active power whichcontributes to the frequency change.

The load-ratio setting means 143 is designed to set the ratio of theconstant impedance load. The value the setting device 143a is to set maybe determined in the following way.

a) The value is determined on the basis of the ratio of theconstant-impedance load and any other load, in consideration of thetotal load of the house that receives power from the sub-station.

b) The value is determined from the ratio of the constant-impedance loadto the any other load, which are always or frequently used in the housethat receive power from the sub-station.

c) The value is switched for time zones, because the ratio of theconstant-impedance load to the any other load differs from time to timedue to the drive pattern of the power plant installed in the house.

To switch the value, a component must be provided to store the datarepresenting the drive pattern.

d) In the case of an inverse tidal current generation, theinverse-current component is not regarded as a constant impedance load,a constant-impedance load is considered to exist in the house only.

Thus, the frequency can be sufficiently changed in consideration of thedifferent degrees in which loads contribute to the frequency change.

(Seventeenth Embodiment)

(Structure)

As shown in FIG. 24, a dummy load 141 is used that comprises a pluralityof inductive loads and a plurality of capacitive loads. Active power Por active power P' is used to determine how many or which of these loadsshould be connected or disconnected. The dummy load 141 is connected ordisconnected by an impedance-inputting breaker 142. Theimpedance-inputting breaker 142 comprises switches 142a to 142f and acontroller 142g. The controller 142g receives either the active power Por the active power P' and a rate V₃₀ of change in the frequency, andeither connects or disconnects the impedance-inputting breaker 142.

(Operation and Advantages)

As described above, the active power supplied to the load is the factordetermining the frequency change in the case where the active power ischanged due to a voltage change and the frequency is changed by a speedcontrol system. On the other hand, the reactance of the dummy load isthe factor determining the frequency change in the case where thefrequency is changed as the dummy load is connected or disconnected. Theactive power may not change greatly as the voltage change and thefrequency may not be sufficiently changed by the speed control system,because the active power supplied to the load is not large. In thiscase, a reactance high enough to change the frequency sufficiently isconnected or disconnected.

Assume that the dummy load 141 (i.e., loads 141a, 141b, 141c, 141d,141e, and 141f) are disconnected from the bus line in FIG. 24. Then, thefrequency change can be increased by connecting the inductive loadsonly, if the frequency change rate V₃₀ detected by the detector 130 hasa positive value. If the frequency change rate V₃₀ has a negative value,the frequency change can be increased by connecting the capacitive loadsonly. Many dummy loads are connected when the active power P or theactive power P' is small. A few dummy load are connected when the activepower P or the active power P' is large. Assume that the gain K isequivalent to the number of dummy loads connected. Then, the relationbetween the active power P or P' and the number of dummy loads to beconnected can be regarded as similar to the relation the active power Pand the gain K have in the embodiment of FIG. 13.

The dummy loads are connected in specific numbers determined asdescribed above if the loads (141a, 141b, 141c, 141d, 141e and 412f)constituting the dummy load 141 have the same capacitance. The loads maydiffer in capacitance. In this case, those of the loads which have largecapacitance are connected if the active power P or P' is small, andthose of the loads which have small capacitance are connected if theactive power P or P' is large.

The frequency can thus be changed sufficiently.

(Eighteenth Embodiment)

In the description of the twelfth to seventeenth embodiments, therevolving type generation system comprising the AC generator 107 andengine 110 has been mainly be explained. Instead, either a generationsystem comprising a DC power supply and an inverter or a reactive powergeneration system may employed. Either generation system may becontrolled to increase the advanced reactive power if the rate of thefrequency change has a positive value, and to increase the delayedreactive power if the rate of the frequency change has a negative value.Then, the same advantages can be achieved as in any one of the twelfthto seventeenth embodiments.

(Nineteenth Embodiments)

(Structure)

As shown in FIG. 25, a frequency (f) detector 221 detects the frequencyfrom the output voltage of an AC generator 207. A frequency-change ratedetector 230 detects a frequency change rate V₃₀ from the frequencydetected.

An excessive frequency-change rate (df/dt) detector 231 detects whetheror not the frequency change rate V₃₀ has exceeded a prescribed value.Upon detecting that the rate V₃₀ has exceeded the prescribed value, thedetector 231 outputs an error signal V₃₁, which is supplied to a faulttrip circuit 220. The fault trip circuit 220 supplies a trip signal to abreaker 206, opening the electric circuit.

A reactive power detector 223 receives the output current of thegenerator 207, which has been detected by a current transformer 212, andthe output voltage of the generator 207, and detects an reactive powertherefrom. An active power detector 226 receives the output current ofthe generator 107, which has been detected by a current transformer 212,and the output voltage of the generator 207, and detects an active powertherefrom.

Meanwhile, an active power regulator (APR) 227 compares the active powerreference P* set by an active power reference (P*) setting device 225with the active power P detected by an active power detector 226. Thedifference between the active power reference and the active power issupplied to a speed governor 211, which controls the of an engine 210.

A first function circuit 232 receives the frequency change rate V₃₀ fromthe frequency-change rate detector 230 and detects a firstvoltage-change reference ΔV*. The voltage-change reference ΔV* willdecrease the output voltage of the generator 207 to increase thefrequency while the frequency change rate has a positive value, or whilethe frequency is increasing. It will increase the output voltage of thegenerator 207 to decrease the frequency while the frequency change ratehas a negative value, or while the frequency is decreasing. The firstvoltage-change reference ΔV* is supplied to an automatic voltageregulator 209.

A second function circuit 233 calculates a second voltage-changereference ΔV2* from the voltage change detected by a voltage changedetector 235, on the basis of the output voltage V of the generator 207that a voltage detector 234 has detected. The second voltage-changereference ΔV2* will decrease the output voltage of the generator 207 toincrease the frequency while the voltage is decreasing. It will increasethe output voltage of the generator 207 to decrease the frequency whilethe voltage is increasing. The signal which the voltage change detector235 detects and which is input to the second function circuit may eithera voltage change ΔV or a voltage change rate dV/dt.

A reactive power regulator (AQR) 224 outputs a voltage reference ΔVQ*for equalizing the reactive power reference Q* supplied from a reactivepower reference setting device 228 and the reactive power detected bythe reactive power detector 223.

An automatic voltage regulator (AVR) 209 receives the voltage referenceV* from a voltage-reference setting device (90R) 2029, the voltagereference ΔVQ* from the reactive power regulator 224, and thevoltage-change reference ΔV* from the first function circuit 232, andvoltage-change reference ΔV2* from the second function circuit 233.Using these inputs, the regulator 209 regulates the field magnet of afiled magnet winding 208 in order to control the output voltage of thegenerator 207.

The active power reference (P*) setting device 225, active powerregulator 227, speed governor 211 and engine 210 constitute a speedcontrol loop. The reactive power reference setting device 228, reactivepower detector 223 and reactive power regulator 224 constitute areactive power control loop. A voltage-reference setting device (90R)229, the voltage reference ΔVQ* output by the reactive power regulator224, voltage-change reference ΔV* supplied from the first functioncircuit 232, voltage-change reference ΔV2* supplied from the secondfunction circuit 233 and automatic voltage regulator 209 constitute avoltage control loop.

(Operation)

The operation of the nineteenth embodiment described above will beexplained, with reference to FIGS. 26 to 29. The active power ΔP andreactive power ΔQ, both supplied to a power system 201 are given asfollows:

    ΔP=P-P.sub.L

    ΔQ=Q-Q.sub.L

where P is the active power output by the generator 207, Q is thereactive power output from the generator 207, P_(L) is the active powera load 205 needs, Q_(L) is the reactive power the load 205 needs, as isshown in FIG. 26.

Here, I is the inductance I between the generator 207 and the system, Vis the voltage applied on the load 205, and f is the frequency.

In most cases, the voltage V on the load 205 and the frequency fscarcely change even if a breaker 203 is opened 2 while ΔP and ΔQ arealmost zero (0). The change in the frequency cannot be detected, and anisolated operation continues.

However, the phase of a system power supply 201 and the phase of theload 205 gradually deviate from each other. If the breaker 203 is closedagain, a great accident may arise. Hence, the breaker 203 cannot beclosed again. This will impair the safety operation of the powerdistribution system.

The voltage during the isolated operation is determined as P=V² /R. Onthe other hand, the frequency f during the isolated operation isdetermined by Q=(V² ωC)-(V² /ωL). The frequency f, in particular,increases when the reactive power the generator 207 supplies advanceswith respect to the reactive power QL the load 205 needs. In this case,the current i_(C) in a capacitor C increases and the inductance currenti_(L) decreases, whereby the reactive power changes to become balanced.

When the reactive power the generator 207 supplies is delayed withrespect to the reactive power QL the load 205 needs, the frequency fdecreases, and the inductance current iL increases. The current i_(C) ina capacitor C decreases, whereby the reactive power changes to becomebalanced.

If ΔP=0 and ΔQ≠0, the frequency f approaches either f1 or f2 whilechanging, as shown in FIG. 27, after the system has been disconnected(t₀). In FIG. 27, f1 is the value the frequency f has when ΔQ isadvanced a little, and f2 is the value the frequency f has when ΔQ isdelayed a little. +Δf and -Δf, both shown in FIG. 15, are levels atwhich the relays 215 to 219 can detect isolated operation.

FIG. 28 is a diagram explaining the operational advantage of theembodiment shown in FIG. 25. In FIG. 28, f is the frequency thefrequency detector 221 has detected, df/dt is the frequency change ratethe frequency-change rate detector 230 has detected, and thevoltage-change reference ΔV* is the output the function circuit 232 hasgenerated.

If f changes as shown in FIG. 28, df/dt will have a waveform whichadvances in phase by 90°.

When df/dt>0, the frequency is increasing. In this case, the functioncircuit 232 outputs a voltage-decreasing command (an advanced reactivepower command), which increases the frequency f further. When df/dt<0,the frequency f is decreasing. In this case, the function circuit 232outputs a voltage-increasing command (a delayed reactive power command),which decreases the frequency f further. This positive feedbackoperation increases the rate of frequency change. The excessivefrequency-change rate detector 231 detects an anomalous frequency or anexcessive rate of frequency change. This makes it possible to detect theisolated operation and protect the system, without using a transferbreaker 238 which is incorporated in the conventional system and whichis an expensive device.

The second function circuit 233 performs positive feedback, furtherlowering the voltage when the voltage is decreasing, and further raisingthe voltage when the voltage is increasing. Cooperating with the firstfunction circuit 232, the second function circuit 233 can increase thefrequency change.

Thanks to such an additional circuit, the frequency change that occursduring the isolated operation can be increased, from the value shown inFIG. 27 to the value shown in FIG. 29. Hence, an anomalous frequency oran excessive rate of frequency change can be detected to detect theisolated operation easily.

(Advantages)

In the nineteenth embodiment described above, df/dt is detected. Avoltage-decreasing command is supplied to the automatic voltageregulator 209 when df/dt>0, and a voltage-increasing command is suppliedto the regulator 209 when df/dt<0. Moreover, a voltage command foramplifying the voltage change is supplied to the voltage regulator 209.The output voltage of the generator 207 is thereby changed, thusincreasing the frequency change. This makes it easy to detect theisolated operation.

(Twentieth Embodiment) (Corresponding to claim 2)

(Structure)

FIG. 30 shows the twentieth embodiment of the invention. The twentiethembodiment differs from the nineteenth embodiment of FIG. 25 in that adetermining circuit 236 and a threshold-value setting device 237 areadditionally provided. The device 237 has set a threshold value for thefrequency change rate. The circuit 236 determines whether or not theoutput of the second function circuit 233 should be input to theautomatic voltage regulator 209, in accordance with the threshold valueset by the threshold-value setting device 237.

The threshold-value setting device 237 sets both a threshold value forthe positive frequency change rate, i.e., the rate at which thefrequency increases, and a threshold value for the negative frequencychange rate, i.e., the rate at which the frequency decreases.

(Operation)

This embodiment is based on the nineteenth embodiment. It is identicalto the nineteenth embodiment in that a voltage change is induced toincrease the frequency change. Even if the system operates well andremains connected, the second function circuit 233 may output avoltage-change reference ΔV2* from the voltage change in the system, inorder to amplify the voltage change. If this happens, the reactive powerwill unnecessarily change between the generation system and the powersystem. Therefore, only the first function circuit 232 is firstoperated, thus changing the frequency. If the frequency change rateexceeds a specific value, the system is considered to have beendisconnected from the power system and the second function circuit 233is then operated.

The voltage-change reference is therefore be prevented fromunnecessarily increasing while the system remains connected. Anexcessive change in reactive power is thereby prevented, and thefrequency change during the isolated operation is increased, from thevalue shown in FIG. 27 to the value shown in FIG. 29. Thus, an anomalousfrequency or an excessive rate of frequency change is detected to detectthe isolated operation easily.

(Advantages)

In the twentieth embodiment described above, df/dt is detected. Avoltage-decreasing command is supplied to the automatic voltageregulator 209 when df/dt>0, and a voltage-increasing command is suppliedto the regulator 209 when df/dt<0. Moreover, a voltage command foramplifying the voltage change is supplied to the voltage regulator 209.The output voltage of the generator 207 is thereby changed, thusincreasing the frequency change. This makes it easy to detect theisolated operation.

(Twenty-First Embodiment)

(Structure)

FIG. 31 shows the twenty-first embodiment of the invention. Thetwenty-first embodiment differs from the twentieth embodiment of FIG. 30in a respect. The circuit 236 determines whether the output of thesecond function circuit 233 should be input to the automatic voltageregulator 209, not only by comparing the frequency change rate with thethreshold value supplied from the threshold-value setting device 237,but also by using the direction of frequency change and the direction ofvoltage change, which are respectively represented by the frequencychange rate and the output signal of the voltage change detector 235.

(Operation)

The present embodiment is based on the nineteenth embodiment. It isidentical to the nineteenth embodiment in that a voltage change isinduced to increase the frequency change.

In the twentieth embodiment described above, only the first functioncircuit 232 is usually operated and both the first function circuit 232and the second function circuit 233 are operated after the frequencychange rate exceeds the threshold value.

A voltage change is induced in the first function circuit 232 whichusually operates, thereby increasing the frequency change. After thesystem has been disconnected from the power system, the frequencyincreases as the voltage lowers, or the frequency decreases as thevoltage rises. Hence, if the frequency changes as the voltage changes inthis manner and if the frequency change rate exceeds the thresholdvalue, the system is considered to have been disconnected from the powersystem. In this case, the second function circuit 233 is operated. Thisprevents the voltage-change reference from unnecessarily increasingwhile the system remains connected. As a result, an excessive change inreactive power is prevented, and the frequency change during theisolated operation is increased, from the value shown in FIG. 27 to thevalue shown in FIG. 29. Thus, an anomalous frequency or an excessiverate of frequency change is detected to detect the isolated operationeasily.

(Advantages)

In the twenty-first embodiment described above, df/dt is detected. Avoltage-decreasing command is supplied to the automatic voltageregulator 209 when df/dt>0, and a voltage-increasing command is suppliedto the regulator 209 when df/dt<0. Further, a voltage command foramplifying the voltage change is supplied to the voltage regulator 209.The output voltage of the generator 207 is thereby changed, thusincreasing the frequency change. This makes it easy to detect theisolated operation.

(Twenty-Second Embodiment)

(Structure)

In the nineteenth embodiment, the second function circuit 233 calculatesa second voltage-change reference ΔV2* from the voltage change detectedby the voltage change detector 235, on the basis of the output voltage Vof the generator 207 that a voltage detector 234 has detected. In thetwenty-second embodiment, the function set in the second functioncircuit 233 has a dead zone. In the dead zone of the function, thevoltage change dv/dt is not detected while remaining within a specificrange, as is illustrated in FIG. 32.

(Operation)

While the system remains connected, the system voltage dominates thevoltage that is detected as the output voltage of the generator 207. Inother words, the voltage detected as the output voltage of the generator207 changes when the system voltage changes. Even while the system isoperating well, the voltage keeps changing due to the fluctuation ofload occurring in the same distribution system, the switching of tapsperformed in the substation, or the like. This means that the secondvoltage-change reference ΔV2* is usually output on the basis of thevoltage change. The reactive power unnecessarily change between thegeneration system and the power system. To avoid this unnecessary changein reactive power, the second function circuit 233 is designed not todetect the voltage change as long as the voltage change remains in aspecific range. Thus, the second function circuit 233 prevents changesof the reactive power while the voltage change remains in that specificrange. Once the isolated operation starts, the first function circuit232 starts operating. When the voltage changes sufficiently, the secondfunction circuit 233 starts operating. The frequency change during theisolated operation is increased, from the value shown in FIG. 27 to thevalue shown in FIG. 29. Thus, an anomalous frequency or an excessiverate of frequency change is detected to detect the isolated operationeasily.

(Advantages)

In the twenty-second embodiment described above, df/dt is detected. Avoltage-decreasing command is supplied to the automatic voltageregulator 209 when df/dt>0, and a voltage-increasing command is suppliedto the regulator 209 when df/dt<0. Further, a voltage command foramplifying the voltage change is supplied to the voltage regulator 209.The output voltage of the generator 207 is thereby changed, thusincreasing the frequency change. This makes it easy to detect theisolated operation.

(Twenty-Third Embodiment)

(Structure)

FIG. 33 shows the twenty-third embodiment of the present invention. Thetwenty-first embodiment differs from the nineteenth embodiment of FIG.19 in that no components equivalent to the function circuits 232 and 233for outputting voltage-change references and the voltage change detector235. It also differs in that a group 241 of capacitors and an inputbreaker 242 are additionally provided.

As shown in FIG. 34, the group 241 consists of power capacitors 242a,241b, 241c . . . The capacitors are connected to a bus line by the inputbreaker 242. The input breaker 242 comprises switches 242a, 242b, 242c,. . . , and a controller 242d. The controller 142d either connects ordisconnects the input breaker 142 in accordance with a signalrepresenting a frequency change rate (df/dt).

(Operation)

As has been explained with reference to FIG. 26, in describing thenineteenth embodiment, during the isolated operation the reactive powerthe generator 207 supplies may advances with respect to the reactivepower QL the load 205 needs. In this case, the frequency f increases,increasing the current iC of the capacitor C and decreasing theinductance current iL. The reactive power changes to be balanced. Duringthe isolated operation, the reactive power the generator 107 suppliesmay delay with respect to the reactive power QL the load 205 needs. Ifso, the frequency f decreases, decreasing the current iC of thecapacitor C and increasing the inductance current iL. The reactive powerchanges to be balanced.

That is, if the reactive power the generator 207 supplies and thereactive power the load needs greatly differ from each other, thefrequency f changes greatly to balance these reactive powers. When thefrequency f changes to balance the inactive powers after the start ofthe isolated operation, it suffices to close or open the capacitors(241a, 241b, . . . ) in order to render the reactive powers unbalanced.

The group 241 of capacitors may be regarded as a power-factor improvingcapacitor provided to the power-factor of the load. Assume that some ofthe capacitors of the group 241 have been connected to the bus line inorder to cancel out the capacitive reactance of the load. While df/dt>0,that is, while the frequency is increasing, the frequency f will furtherincrease if the capacitors are disconnected from the bus line. Whiledf/dt<0, that is, the frequency is decreasing, the frequency f willfurther decrease if the capacitors are connected to the bus line. Thus,the change in the frequency is enhanced. The excessive frequency-changerate detector 231 can therefore detect an anomalous frequency or anexcessive rate of frequency change. This makes it possible to detect theisolated operation, without using a transfer breaker which is used inthe conventional system and which is an expensive device.

Thanks to such an additional circuit, the frequency change that occursduring the isolated operation can be increased, from the value shown inFIG. 27 to the value shown in FIG. 29. Hence, an anomalous frequency oran excessive rate of frequency change can be detected to detect theisolated operation easily.

(Advantages)

In the twenty-third embodiment described above, df/dt is detected. Thepower capacitors are connected or disconnected in accordance with thepolarity of the df/dt detected. The frequency change is therebyincreased, making it easy to detect the isolated operation.

(Twenty-Fourth Embodiment)

(Structure)

FIG. 35 shows the twenty-fourth embodiment of the invention. Thetwenty-fourth embodiment differs from the nineteenth embodiment of FIG.19 in that no components equivalent to the second function circuit 233,voltage detector 234 and voltage change detector 235. It also differs inthat a group 241 of capacitors and an input breaker 242 are additionallyprovided. The group 41 of capacitors and the input breaker 242 areidentical in structure to those shown in FIG. 34, respectively.

(Operation and Advantages)

As has been described in explaining the nineteenth and twenty-thirdembodiments, the voltage-change reference ΔV* output from the functioncircuit 232 has the function of increasing the frequency change further.The frequency change is also increased by disconnecting the group 241 ofcapacitors.

Thanks to the reference ΔV* and the disconnection of the group 241, thefrequency change that occurs during the isolated operation can beincreased, from the value shown in FIG. 27 to the value shown in FIG.29. An anomalous frequency or an excessive rate of frequency change canthereby be detected to detect the isolated operation easily.

(Twenty-Fifth Embodiment)

(Structure)

The twenty-fifth embodiment differs from the sixth embodiment shown inFIGS. 34 and 35 in that a threshold-value setting device 237 isadditional provided. The device 237 is designed to input a signal to thecontroller 2422d incorporated in the input breaker 242a. The signalrepresents a threshold value for the frequency change rate.

The threshold-value setting device 237 sets both a threshold value forthe positive frequency change rate, i.e., the rate at which thefrequency increases, and a threshold value for the negative frequencychange rate, i.e., the rate at which the frequency decreases. Thus, thedevice 237 serves to connect or disconnect the power capacitors, notonly in accordance with which polarity the frequency change rate has,but also in accordance with whether the frequency change rate hasexceeded the threshold value.

(Operation and Advantages)

As has been described in explaining the nineteenth and twenty-thirdembodiments, the voltage-change reference ΔV* output from the functioncircuit 232 has the function of increasing the frequency change further.The frequency change is also increased by disconnecting the group 241 ofcapacitors.

The reference ΔV* and the disconnection of the group 241 are utilized inthe same way as in the twenty-fourth embodiment. However, the powercapacitors are not so frequently connected and disconnected. Rather, thefrequency is changed by using the function circuit only. When thefrequency change rate exceeds the threshold value, it is considered thatthe system has been disconnected from the power system. Then, the powercapacitors are connected or disconnected. Hence, the frequency changethat occurs during the isolated operation can be increased, from thevalue shown in FIG. 3 to the value shown in FIG. 29, without changingthe reactive power frequently. An anomalous frequency or an excessiverate of frequency change can thereby be detected to detect the isolatedoperation easily.

(Twenty-Sixth Embodiment)

(Structure)

FIG. 37 shows the twenty-sixth embodiment of the invention. Thetwenty-sixth embodiment differs from the nineteenth embodiment of FIG.19 in three respects. First, no component equivalent to the secondfunction circuit 233 is provided. Second, a group 241 of capacitors andan input breaker 242 are additionally provided. Third, a threshold-valuesetting device 237 is additionally provided to set a threshold value ofthe frequency change rate.

As shown in FIG. 38, the threshold-value setting device 237 inputs athreshold value to the controller 232d provided in the input breaker242, in the same way as in the twenty-fifth embodiment. The thresholdvalue is compared with the frequency change, thereby to determinewhether the power capacitors have been connected or disconnected. Asshown in FIG. 38, the signal representing the voltage change andgenerated by the voltage detector 234 and the voltage change detector235 is input to the controller 242d. This signal is used to determinewhether the power capacitors have been connected or disconnected.

(Operation and Advantages)

As has been described in explaining the nineteenth and twenty-thirdembodiments, the voltage-change reference ΔV* output from the functioncircuit 232 has the function of increasing the frequency change further.The frequency change is also increased by disconnecting the group 241 ofcapacitors.

The reference ΔV* and the disconnection of the group 241 are utilized inthe same way as in the twenty-fourth embodiment. However, the powercapacitors are not so frequently connected and disconnected. Rather, thefrequency is changed by using the function circuit only. Once the systemis disconnected from the system power supply, the frequency increases asthe voltage rises, and decreases as the voltage lowers. Hence, it thefrequency changes in either way and if the frequency change rate exceedsthe threshold value, it is detected that the system has beendisconnected from the power system. Then, the power capacitors areconnected or disconnected.

Therefore, the frequency change that occurs during the isolatedoperation can be increased, from the value shown in FIG. 27 to the valueshown in FIG. 29, without changing the reactive power frequently. Ananomalous frequency or an excessive rate of frequency change can therebybe detected to detect the isolated operation easily.

(Twenty-Seventh Embodiment)

The frequency will not change while the system remains connected, evenif the power capacitors are connected or disconnected in order toincrease the frequency change. Connection or disconnection of the powercapacitors induces a change in the reactive power. This function of thepower capacitors can be applied to the improve the power factor. Ifappropriately adjusted before the power capacitors are connected ordisconnected in accordance with the frequency change rate, the powerfactor will have an undesirable value. If the frequency change cannot beincreased, the system is considered to be connected, not performingisolated operation. In this case, the influence of the power capacitorseither connected or disconnected is nullified, thereby changing thepower factor back to a desirable value.

(Twenty-Eighth Embodiment)

In the description of the nineteenth to twenty-seventh embodiments, therevolving type generation system comprising the AC generator 207 andengine 210 has been mainly explained. Instead, either a staticgeneration system comprising a DC power supply and an inverter or areactive power generation system may be employed. Either generationsystem may be controlled to increase the advanced reactive power if therate of the frequency change has a positive value, and to increase thedelayed reactive power if the rate of the frequency change has anegative value. Then, the same advantages can be achieved as in any oneof the twelfth to seventeenth embodiments.

Industrial Applicability

As has been described, the present invention can provide an apparatusfor protecting a non-utility generation system, which has functioncircuits so designed to make it possible to detect the isolatedoperation of the generation system within a short time even if it ismost difficult to detect it because neither active power nor reactivepower is supplied to the system, which can maintain the reactive powerchange (voltage change) within an appropriate range while the generationsystem remains connected, and which can operate at high speed, in highstability and with high reliability even if a number of generators areoperating or revolving type generators and static type generators areoperating together.

Moreover, the present invention can provide an apparatus for protectinga non-utility generation system, which can reliably detects the isolatedoperation of the non-utility generation system connected to an uppersubstation by the interconnection system, without using a transferbreaker which is expensive.

What is claimed is:
 1. An apparatus for protecting a non-utilitygeneration system which is interconnected by a breaker to a power systemand which has a voltage control system, said apparatus comprising:afrequency detector for detecting an output frequency of the non-utilitygeneration system; a frequency-change rate detector for detecting a rateat which the frequency detected by the frequency detector changes;arithmetic means for calculating a change reference for a voltage orreactive power output by the non-utility generation system, from thefrequency change rate detected by the frequency-change rate detector;first control means for increasing an advanced reactive power of thenon-utility generation system or decreasing the output voltage of thenon-utility generation system, when it is determined from the changereference that the frequency change rate has a positive value; secondcontrol means for increasing a delayed reactive power of the non-utilitygeneration system or increasing the output voltage of the non-utilitygeneration system, when the frequency change rate has a negative value;gain adjusting means for adjusting a gain of a power control section ofthe non-utility generation system, in accordance with the frequencychange rate; and protecting means for detecting a frequency change whichincreases as the voltage of the non-utility generation system changes,and for disconnecting the non-utility generation system from the systembus line in accordance with the frequency change detected.
 2. Anapparatus according to claim 1, wherein the gain adjusting means hasgain setting means for setting a gain in the power control section ofthe non-utility generation system, said gain being high when thefrequency change rate falls in an extremely low range, being low whenthe frequency change rate falls in an intermediate range, and being arelatively high when the frequency change rate falls in a high range. 3.An apparatus according to claim 1, wherein the gain adjusting means hasmeans for switching the gain of the power control section in accordancewith the frequency change rate and/or switching a limited valuecorresponding to the change reference.
 4. An apparatus according toclaim 1, wherein the non-utility generation system comprises a revolvinggeneration system.
 5. An apparatus according to claim 1, wherein thenon-utility generation system comprises a static generation system. 6.An apparatus for protecting a non-utility generation system which isinterconnected by a breaker to a power system, said apparatuscomprising:a frequency change detector for detecting changes in anoutput frequency of the non-utility generation system; control means foroutputting a control signal to the non-utility generation system,thereby to control a reactive power, a preset output voltage, an outputvoltage, an output voltage phase or an output current phase of thenon-utility generation system; reactive power-change rate detector fordetecting a rate at which a reactive power of the non-utility generationsystem changes; voltage-change rate detecting means for detecting a rateat which an output-voltage reference of the non-utility generationsystem changes; frequency-change increasing means for changing an outputof the non-utility generation system, thereby to increase the frequencychange, when the reactive power-change rate detecting means and thevoltage-change rate detecting means detect a change in the frequency ofthe non-utility generation system; and operating mode setting means forsetting an operating mode of the non-utility generation system when thereactive power-change rate decreases as the frequency-change increasingmeans increases the frequency change.
 7. An apparatus according to claim6, wherein the operating mode setting means has stop/disconnect meansfor stopping the non-utility generation system which gives adisconnection command to the breaker, or for disconnecting thenon-utility generation system from the system bus line, when thefrequency-change increasing means increases the frequency change,thereby decreasing the reactive power-change rate.
 8. An apparatusaccording to claim 6, wherein the operating mode setting means comprisesisolated operation means for disconnecting the non-utility generationsystem from the power system and turning off the frequency-changeincreasing means, thereby to cause the non-utility generation system toperform isolated operation, when the frequency-change increasing meansincreases the frequency change, thereby decreasing the reactivepower-change rate.
 9. An apparatus according to claim 6, wherein thenon-utility generation system comprises a revolving generation system.10. An apparatus according to claim 6, wherein the non-utilitygeneration system comprises a static generation system.
 11. An apparatusfor protecting a non-utility generation system which is interconnectedby a breaker to a power system and which has a voltage control system,said apparatus comprising:a frequency detector for detecting an outputfrequency of the non-utility generation system; a frequency-change ratedetector for detecting a rate at which the frequency detected by thefrequency detector changes; low-speed response reactive power controlmeans for detecting a reactive power of the non-utility generationsystem, said low-speed response reactive power control means beingcontrolled by a first voltage reference to change the reactive power toa desired value; high-speed response reactive power control meanscontrolled by a reactive-power change reference and a second voltagechange, said reactive-power change reference advancing the reactivepower when said frequency-change rate detector detects that thefrequency change rate has a positive value, and delaying the reactivepower when said frequency-change rate detector detects that thefrequency change rate has a negative value, and said second voltagechange having been obtained by comparing the reactive power and thereactive power change; voltage control means for controlling an outputvoltage of the non-utility generation system in accordance with a thirdvoltage reference obtained from the first voltage reference forcontrolling the low-speed response reactive power control means and thesecond voltage reference for controlling the high-speed responsereactive power control means; and protective means for disconnecting thenon-utility generation system from a bus line in accordance with thereactive power change and also with the second voltage reference forcontrolling the high-speed response reactive power control means.
 12. Anapparatus according to claim 11, wherein the non-utility generationsystem comprises a revolving generation system.
 13. An apparatusaccording to claim 11, wherein the non-utility generation systemcomprises a static generation system.
 14. An apparatus according toclaim 11, wherein the non-utility generation system comprises a staticpower converter having a circuit which delays the reactive power controlor the voltage change control.
 15. An apparatus according to claim 11,wherein the non-utility generation system comprises a static powerconverter having a circuit which delays the reactive power control orthe voltage change control.
 16. An apparatus for protecting anon-utility generation system which is interconnected a breaker to apower system, said apparatus comprising:a frequency detector fordetecting an output frequency of the non-utility generation system; afrequency-change rate detector for detecting a rate at which thefrequency detected by the frequency detector changes; a function circuitfor calculating a voltage-change reference from the frequency changerate detected by the frequency-change rate detector and for controllingthe non-utility generation system to increase an advanced reactive powerof the non-utility generation system or decrease the output voltage ofthe non-utility generation system when it is determined from the voltagechange reference that the frequency change rate has a positive value,and to increase a delayed reactive power of the non-utility generationsystem or increase the output voltage of the non-utility generationsystem when it is determined from the voltage change reference that thefrequency change rate has a negative value; an active power detector fordetecting an active power of the non-utility generation system;voltage-change reference correcting means for increasing thevoltage-change reference output from the function circuit, thereby tosufficiently increase the output frequency of the non-utility generationsystem when the active power detected by the active power detect or issmall; and a protective device for detecting a frequency changeincreasing as the voltage of the non-utility generation system changesand for disconnecting the non-utility generation system from a bus line.17. An apparatus according to claim 16, wherein the non-utilitygeneration system comprises a section having a static DC power supplyand a power converter.
 18. An apparatus according to claim 16, whereinthe non-utility generation system comprises a section having a static DCpower supply and a device for compensating for reactive power.
 19. Anapparatus for protecting a non-utility generation system which isinterconnected by a breaker to a power system, said apparatuscomprising:a frequency detector for detecting an output frequency of thenon-utility generation system; a frequency-change rate detector fordetecting a rate at which the frequency detected by the frequencydetector changes; a group of capacitors which are power capacitors;input breaker means for disconnecting the group of capacitors from thenon-utility generation system when the frequency change rate detected bythe frequency-change rate detector has a positive value and connectingthe group of capacitors to the non-utility generation system when thefrequency change rate detected by the frequency-change rate detector hasa negative value; and a protective device for detecting a frequencychange increased as the reactive power changes when the group ofcapacitors is connected or disconnected, and for disconnecting thenon-utility generation system from a base line in accordance with thefrequency change detected.
 20. An apparatus according to claim 19,wherein the non-utility generation system comprises a section having astatic DC power supply and a power converter.
 21. An apparatus accordingto claim 19, wherein the non-utility generation system comprises asection having a static DC power supply and a device for compensatingfor reactive power.
 22. An apparatus for protecting a non-utilitygeneration system which is interconnected by a breaker to a powersystem, said apparatus comprising:a frequency detector for detecting anoutput frequency of the non-utility generation system; afrequency-change rate detector for detecting a rate at which thefrequency detected by the frequency detector changes; a function circuitfor calculating a voltage-change reference from the frequency changerate detected by the frequency-change rate detector and for controllingthe non-utility generation system to increase an advanced reactive powerof the non-utility generation system or decrease the output voltage ofthe non-utility generation system when it is determined from the voltagechange reference that the frequency change rate has a positive value,and to increase a delayed reactive power of the non-utility generationsystem or increase the output voltage of the non-utility generationsystem when it is determined from the voltage change reference that thefrequency change rate has a negative value; a group of capacitors whichare power capacitors, input breaker means for disconnecting the group ofcapacitors from the non-utility generation system when the frequencychange rate detected by the frequency-change rate detector has apositive value and connecting the group of capacitors to the non-utilitygeneration system when the frequency change rate detected by thefrequency-change rate detector has a negative value; and a protectivedevice for detecting a frequency change increased as the reactive powerchanges when the group of capacitors is connected or disconnected, andfor disconnecting the non-utility generation system from a base line inaccordance with the frequency change detected.
 23. An apparatusaccording to claim 22, wherein the group of capacitors is disconnectedfrom the non-utility generation system when the frequency change ratehas a positive value and exceeds a preset positive value, and isconnected to the non-utility generation system when the frequency changerate has a negative value and exceeds a preset negative value.
 24. Anapparatus according to claim 22, wherein the non-utility generationsystem comprises a section having a static DC power supply and a powerconverter.
 25. An apparatus according to claim 22, wherein thenon-utility generation system comprises a section having a static DCpower supply and a device for compensating for reactive power.